RE: [EXT] Re: [PATCH] i2c: pca954x: Add property to skip disabling PCA954x MUX device

[Biwen Li]

> 
> On 2019-09-29 12:36, Biwen Li wrote:
> > On some Layerscape boards like LS2085ARDB and LS2088ARDB, input
> > pull-up resistors on PCA954x MUX device are missing on board, So, if
> > MUX are disabled after powered-on, input lines will float leading to
> > incorrect functionality.
> 
> Hi!
> 
> Are you saying that the parent bus of the mux is relying on some pull-ups inside
> the mux?
Yes, as follows:
													VCC
													-------
													  |-------------------
                                                      |            |
													  \            \
													  /10K resister  / 10k resister
													  \            \
													  |            |
                                                      |            |
   I2C1_SCL  ------------------------                         I2C1_SCL       |       ----------------------
--------------------|SCL           |                      ----------------------------------------|SCL          |
   I2C1_SDA  |   PCA9547   |                        I2C1_SDA       |       |    PCA9547  |          
--------------------|SDA           |                      ----------------------------|-----------|SDA         |
             ------------------------                                                ----------------------
  --wrong design(need software fix as above or hardware fix)--              --proper design--
> 
> > Hence, PCA954x MUX device should never be turned-off after power-on.
> >
> > Add property to skip disabling PCA954x MUX device if device tree
> > contains "i2c-mux-never-disable"
> > for PCA954x device node.
> >
> > Errata ID: E-00013 on board LS2085ARDB and LS2088ARDB (Board revision
> > found on Rev.B, Rev.C and Rev.D)
> 
> I think you should follow the example of the i2c-mux-gpio driver and implement
> the idle-state property instead.
> 
> That is a lot more consistent, assuming it solves the problem at hand?
Got it, thanks, I will try it.
> 
> >
> > Signed-off-by: Biwen Li <biwen.li@xxxxxxx>
> > ---
> >  drivers/i2c/muxes/i2c-mux-pca954x.c | 33
> > +++++++++++++++++++++++++----
> >  1 file changed, 29 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/i2c/muxes/i2c-mux-pca954x.c
> > b/drivers/i2c/muxes/i2c-mux-pca954x.c
> > index 923aa3a5a3dc..ea8aca54d572 100644
> > --- a/drivers/i2c/muxes/i2c-mux-pca954x.c
> > +++ b/drivers/i2c/muxes/i2c-mux-pca954x.c
> > @@ -93,6 +93,7 @@ struct pca954x {
> >       struct irq_domain *irq;
> >       unsigned int irq_mask;
> >       raw_spinlock_t lock;
> > +     u8 disable_mux;         /* do not disable mux if val not 0 */
> 
> Awful number of negations there. The name is also backwards given that a
> non-zero value means that the mux should *not* be disabled. I would have
> reused the name from the binding.
> 
>         bool never_disable;
> 
> A bit less confusing...
Got it,thanks, I will let it clear in v2.
> 
> >  };
> >
> >  /* Provide specs for the PCA954x types we know about */ @@ -258,6
> > +259,11 @@ static int pca954x_deselect_mux(struct i2c_mux_core *muxc,
> u32 chan)
> >       struct i2c_client *client = data->client;
> >       s8 idle_state;
> >
> > +     if (data->disable_mux != 0) {
> 
> Please drop " != 0" and use the variable as a truth value. More instances below...
Got it, I will correct it in v2.
> 
> > +             data->last_chan = data->chip->nchans;
> > +             return pca954x_reg_write(muxc->parent, client,
> data->disable_mux);
> > +     }
> > +
> >       idle_state = READ_ONCE(data->idle_state);
> >       if (idle_state >= 0)
> >               /* Set the mux back to a predetermined channel */ @@
> > -462,16 +468,32 @@ static int pca954x_probe(struct i2c_client *client,
> >               }
> >       }
> >
> > +     /* Errata ID E-00013 on board LS2088ARDB and LS2088ARDB:
> > +      * The point here is that you must not disable a mux if there
> > +      * are no pullups on the input or you mess up the I2C. This
> > +      * needs to be put into the DTS really as the kernel cannot
> > +      * know this otherwise.
> > +      */
> > +
> > +     data->disable_mux = np &&
> > +             of_property_read_bool(np, "i2c-mux-never-disable") &&
> 
> i2c-mux-never-disable needs to be documented. However see above for my
> point that you should do it like the i2c-mux-gpio driver. Any usage of idle-state
> still needs to be documented for pca954x binding.
Got it, I will update the document in v2.
> 
> > +             data->chip->muxtype == pca954x_ismux ?
> > +             data->chip->enable : 0;
> 
> Why do you not allow never-disable for switches?
Currently, the hardware bug is on pca9547(LS2085ARDB and LS2088ARDB not populate resistors for pca9547), 
muxtype of pca9547 is 'pca954x_ismux' 
> 
> Cheers,
> Peter
> 
> > +
> >       /* Write the mux register at addr to verify
> >        * that the mux is in fact present. This also
> >        * initializes the mux to disconnected state.
> >        */
> > -     if (i2c_smbus_write_byte(client, 0) < 0) {
> > +     if (i2c_smbus_write_byte(client, data->disable_mux) < 0) {
> >               dev_warn(dev, "probe failed\n");
> >               return -ENODEV;
> >       }
> >
> > -     data->last_chan = 0;               /* force the first selection */
> > +     if (data->disable_mux != 0)
> > +             data->last_chan = data->chip->nchans;
> > +     else
> > +             data->last_chan = 0;               /* force the first
> selection */
> > +
> >       data->idle_state = MUX_IDLE_AS_IS;
> >
> >       idle_disconnect_dt = np &&
> > @@ -531,8 +553,11 @@ static int pca954x_resume(struct device *dev)
> >       struct i2c_mux_core *muxc = i2c_get_clientdata(client);
> >       struct pca954x *data = i2c_mux_priv(muxc);
> >
> > -     data->last_chan = 0;
> > -     return i2c_smbus_write_byte(client, 0);
> > +     if (data->disable_mux != 0)
> > +             data->last_chan = data->chip->nchans;
> > +     else
> > +             data->last_chan = 0;
> > +     return i2c_smbus_write_byte(client, data->disable_mux);
> >  }
> >  #endif
> >
> >